In certain airborne wide area surveillance missions, particularly on board a small unmanned aerial vehicle (“UAV”), there are sensors positioned on the UAV that include infrared imaging optical systems used to image light in the infrared optical spectrum (from about 2 micrometers to about 7 micrometers), which is produced by hot objects such as engines, the human body, or missiles fired at the UAV, onto detectors. The detectors then convert the light into electrical signals that can be further analyzed.
These sensors may use an inverse-telephoto lens system, also known as a fisheye lens, because it allows the field of view to be very large. Further, the detectors in these systems must be cryogenically cooled, while the optical system is not. Therefore, these optical systems have external pupils (or stops) for the location of cold shield that surrounds only the cryogenically cooled detectors.
Previously, inverse telephoto optical systems have had physical length to effective focal length ratios in the 8× to 10× range. These are too large to fit the anticipated 10 inch to 12 inch diameter gimbals that can be employed on small UAV platforms. Further, these known systems employ large diameter lenses that are expensive to manufacture.
The size of the optical system is as important as the field of view (“FOV”), aperture (or stop), and focal length that the optical system is capable of providing. The availability of large two-dimension staring arrays with pixel counts up to 8,000 by 8,000 further emphasizes the need for very compact optical systems.